1. Field of the Invention
The invention relates to an exhaust gas purification system for an internal combustion engine, which includes an exhaust gas recirculation apparatus, and a control method for the same.
2. Description of the Related Art
An exhaust gas purification system that includes an exhaust gas recirculation apparatus (hereinafter, referred to as “EGR apparatus”) and a NOx storage reduction catalyst (hereinafter, may be simply referred to as “NOx catalyst”) is available as a technology for decreasing the amount of nitrogen oxide (NOx) discharged from an internal combustion engine into the atmosphere.
The EGR apparatus decreases the amount of NOx generated in the internal combustion engine, by introducing part of exhaust gas into an intake system for the internal combustion engine, thereby decreasing the combustion temperature of air-fuel mixture in a combustion chamber.
When the concentration of oxygen in exhaust gas that flows into the NOx catalyst is high, the NOx catalyst stores (absorbs, adsorbs) NOx in the exhaust gas. When the concentration of oxygen in the exhaust gas that flows into the NOx catalyst decreases, and a reducing agent is present, the NOx catalyst reduces stored NOx. When the amount of NOx stored in the NOx catalyst increases, purification performance of the NOx catalyst decreases. Therefore, a rich spike control is executed. The rich spike control reduces NOx stored in the NOx catalyst by supplying a reducing agent (for example, fuel) from a reducing agent supply valve provided upstream of the NOx catalyst into the exhaust gas to temporarily decrease the air-fuel ratio of the exhaust gas that flows into the NOx catalyst.
It is required to dispose the NOx catalyst close to the internal combustion engine to suppress a decrease in the temperature of the NOx catalyst. Japanese Patent Application Publication No. 2000-8835 (JP-A-2000-8835) describes an exhaust gas purification system that includes an EGR apparatus that recirculates part of exhaust gas passing through a portion of an exhaust passage, which is located downstream of a NOx catalyst, to an intake system.
However, if the rich spike control for the NOx catalyst is executed when the exhaust gas is recirculated, a large amount of the reducing agent that has flown out of the NOx catalyst may flowing into the intake system for the internal combustion engine. As a result, torque may be sharply decreased (i.e., a so-called torque shock may occur) due to deterioration of the combustion state of air-fuel mixture, which may result in deterioration of drivability.